Preparation and purification of diborane

ABSTRACT

BF 3 CO 2  or both are removed from a mixture containing these gases with B 2 H 6  by contacting the mixture with an inorganic hydroxide such as LiOH. B 2 H 6  is synthesized by contacting BF 3  with KBH 4 .

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of application Ser. No.09/662,015 filed Sept. 14, 2000, now U.S. Pat. No. 6,517,796, and is adivisional application of application Ser. No. 09/288,931 filed on Apr.9, 1999, now U.S. Pat. No. 6,165,434, which claims the benefit of U.S.Provisional Patent Application 60/081,249, filed Apr. 9, 1998.

BACKGROUND OF THE INVENTION

The present invention relates to production and purification ofdiborane. Diborane (B₂H₆) is a flammable gas which is used as a p typedopant in semiconductors, and is also used in boron-phosphate-silicateglass forming. Diborane forms a wide variety of complexes with lewisbases such as borane-tetrahydrofuran, borane dimethyl sulfide and avariety of amine boranes. These compounds are widely used as selectivereducing agents in synthesis of pharmaceuticals, fine organic chemicalsand electroless metal plating baths.

At room temperature, diborane slowly decomposes to higher boranes withtheir physical state ranging from gaseous to solid. This causes processvariations and equipment malfunctions. In order to reduce decomposition,diborane is sometimes shipped as a mixture with a blanket gas or at lowtemperature, such as at dry ice temperature. Another way to overcome thedecomposition problem is to employ point-of-use diborane generation.However, the difficulties encountered with present synthesis andpurification processes have inhibited point-of-use diborane generation.

Numerous possible methods of diborane synthesis have been published. Themost typical and commercially used synthesis method is the reaction ofsodium borohydride with boron trifluoride in ether solvents such asdiglyme. Because this process uses highly inflammable solvents, itrequires significant safety precautions. Further, diborane complexeswith solvents. Such complexes make it difficult to purify the diborane.

A preferred dry process for diborane synthesis is described in U.S. Pat.No. 4,388,284. This process involves reaction of lithium or sodiumborohydride with boron trifluoride (BF₃) in the absence of a solvent. Asa preferred method, the patent describes condensing gaseous borontrifluoride at liquid nitrogen temperature onto sodium borohydride, thenwarming the resultant mixture to a reaction temperature of 0 to 50° C.and holding the mixture at the reaction temperature for 4 to 12 hours.The process yields a mixture containing about 95% diborane and alsocontaining unreacted boron trifluoride. Under similar conditions,reaction of lithium borohydride with boron trifluoride is sluggish andgives poor yield.

While the dry process provides diborane free from solvent contamination,the product contains significant amount of unreacted boron trifluoride.To achieve high purity diborane, tedious distillation is required toseparate the diborane from the boron trifluoride. The process is slowfor commercial production and is a batch process. Based uponthermodynamic considerations, the reaction of lithium borohydride withboron trifluoride should be more favored than the comparable reactionwith sodium borohydride, but the observations set forth in the '284patent indicate that the reaction involving lithium borohydride does notwork well in practice.

SUMMARY OF THE INVENTION

One aspect of the present invention provides methods for treatingmixtures containing diborane and boron trihalides such as borontrifluouride by contacting the mixture with a reagent compositionincluding one or more inorganic hydroxides. This aspect of the inventionincorporates the discovery that inorganic hydroxides selectivelyscavenge boron trihalides, and particularly BF₃, from gas mixturescontaining diborane. For example, the reagent composition may includeone or more alkali metal hydroxides such as sodium, potassium or lithiumhydroxides; alkaline earth hydroxides such as beryllium, calcium,strontium and barium hydroxides; ammonium hydroxide; and transitionmetal hydroxides. Mixtures of these materials may be employed.Desirably, the reagent composition includes a substantial amount of theinorganic hydroxide, i.e. more than 10%, and preferably more than 20%hydroxides. Preferably, the composition is predominantly composed of thehydroxide or hydroxides, i.e., the reagent composition contains morethan 50% mole fraction hydroxides. Most preferably, the reagentcomposition consists essentially of the hydroxide or hydroxides. Thereagent composition typically is present in solid form, such as powder,pellets, granules or a coating on an inert support such as alumina orsilica. The reagent composition may be pretreated by holding it at anelevated temperature prior to use, as, for example, by baking in aninert atmosphere prior to use.

The temperature in the contacting step preferably is room temperature(about 20° C.) or below, and more preferably about 0° C. or below.Temperatures below about −20° C., and desirably below about −40° C., areeven more preferred. The use of such low temperatures minimizesdecomposition of diborane in the process. Most preferably, thediborane-containing mixture is in the gaseous state when contacted withthe reagent composition. Therefore, the temperature in the contactingstep desirably is above the boiling temperature of diborane at thepressure employed. Stated another way, the prevailing pressure in thecontacting step is below the equilibrium vapor pressure of diborane atthe temperature employed for the contacting step. The boilingtemperature of diborane is about −92° C. at atmospheric pressure, andhence the temperature in the contacting step desirably is above about−92° C. if the contacting step is performed at about atmosphericpressure. Dry ice temperature (about −80° C.) is particularly preferred.

The time of contact between the mixture and the reagent composition maybe a few seconds to a few hours, although very short contact times of afew seconds are more preferred. The contacting step can be performedbatchwise or, preferably, on a continuous basis, by passing the mixturecontinuously through a vessel containing the reagent composition. Theflow rate through the vessel, and the proportions of the vessel andamount of reagent composition can be selected to provide any desiredcontact time. Desirably, the purifying process, and particularly thecontacting step, are performed at a location where the purified diboraneis to be used, and the diborane is purified about 4 hours or less beforeit is used. Most preferably, the diborane is purified immediately beforeit is used.

Although this aspect of the invention has been summarized above inconnection with purification of diborane, the process also can beapplied to purification of other inorganic hydrides, and removal ofinorganic halides other than boron trihalides such as BF₃. Thus, processis applicable to remove inorganic halides from inorganic hydridesselected from the group consisting of diborane, silane (SiH₄), germane(GeH₄), phosphine (PH₃), arsine (AsH₃), stibine (SbH₃) and mixturesthereof. Desirably, the inorganic halides which is or are removed areselected from the group consisting of BF₃, SiF₄, GeF₄, PF₃, PF₅, AsF₃,AsF₅, SbF₃, SbF₅ and mixtures thereof.

A further aspect of the invention includes the realization thatcontacting the diborane-containing mixture with a hydroxide-containingreagent mixture will also serve to remove carbon dioxide if carbondioxide is present in the mixture. Thus, processes according to thisaspect of the invention include the steps of contacting a mixturecontaining diborane or other inorganic hydride as discussed above andcarbon dioxide with a hydroxide-containing reagent. The processconditions may be as discussed above in connection with removal ofhalides. Where the gas mixture contains both halides and carbon dioxide,both can be removed in a single contacting step.

Yet another aspect of the invention provides methods of synthesizingdiborane comprising reacting a borohydride reactant including potassiumborohydride (KBH₄) with a boron trihalide, most preferably BF₃, tothereby form a reaction product. The reaction desirably is performed ata reaction temperature of about −130° C. to about 20° C. The reactantdesirably includes at least 20% potassium borohydride, and preferablyconsists essentially of potassium borohydride or includes potassiumborohydride together with sodium borohydride (NaBH₄). The reacting stepdesirably is performed in the absence of a solvent and thus is referredto herein as a “dry” process. The reaction desirably is performed bycontinuously passing the boron trihalide, in gaseous form, as by passingthe boron trihalide through a vessel containing the borohydride reactantin solid form. The reaction can also be performed in batchwise fashion,as by condensing the borohalide on the reactant in a vessel and thenwarning the vessel, reactant and borohalide. This aspect of theinvention incorporates the realization that higher conversion of borontrifluoride to diborane is achieved by reacting it with potassiumborohydride than with either lithium or sodium borohydride. The reactionwith potassium borohydride is especially favored at the preferredtemperatures of about −130 to about 20° C. Most desirably, the reactionconditions are selected so that liquid BF₃ is present in contact withthe borohydride reactant during at least part of the reaction. Thus, BF₃desirably condenses on the borohydride reagent.

Still further aspects of the invention provide apparatus for performingthe processes discussed above. Thus, one aspect of the inventionprovides a purifier to selectively scavenge inorganic halides such asBF₃ and carbon dioxide from diborane-containing or other inorganichydride mixtures, and also provides a diborane generation systemincluding such a purifier. Another aspect of the invention provides agenerator for making diborane using the potassium borohydride reactiondiscussed above, which may also include a purifier as discussed above.Apparatus according to these aspects of the invention may be installedat the point of use, and desirably is connected directly todiborane-using process equipment for continuous or batchwise transfer ofthe diborane made or purified in the apparatus into the diborane-usingequipment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of apparatus in accordance with one embodiment ofthe invention.

FIG. 2b is an infrared spectrum of a mixture including diborane and BF₃.

FIG. 2a is an infrared spectrum of the mixture of FIG. 2b afterpurification in accordance with one embodiment of the invention.

FIG. 3b is an infrared spectrum of a another mixture including diboraneand BF₃.

FIG. 3a is an infrared spectrum of the mixture of FIG. 3b afterpurification in accordance with another embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Apparatus according to one embodiment of the invention includes a vacuumtight stainless steel closed system. A gas cylinder S having a pressureregulating valve V0 is connected with a reaction cylinder R containingthe borohydride reagent in solid form. Reaction cylinder R has an inletvalve V1 and an outlet valve V2 on opposite ends. The reaction cylinderis jacketed to facilitate cooling and low temperature reaction. Theoutlet valve V2 of cylinder R is connected to a tee T1. One branch oftee T1 is connected to a purifier cylinder or bubbler P having inletvalve V3 and outlet valve V4. The purifier cylinder contains a hydroxidereagent in solid form. The outlet valve of the purifier cylinder isconnected through another tee T2 and valve V5 to a gas receiving deviceF. Device F may be a receiver cylinder for collecting the diborane, ormay be a piece of process equipment which consumes diborane, such as asemiconductor thin film deposition system or other reactor.

A bypass valve V6 and bypass line B connects tee T1 with a further teeT3, which is connected to tee T2. The remaining branch of tee T3 isconnected though a further valve V7 to a vacuum manifold M equipped withpressure and vacuum gauges. The manifold is equipped with a wasteconnection W, connected to a pump and waste gas scrubber. The manifoldis also connected to an infrared spectrophotometer cell I, andadditional valves V8, V9, and V10 are provided. Flow meters and/orcontrollers (not shown) may be provided at source S, at valve V3 and atother locations where desired for monitoring the process to achieve therequired capacity generator. The bypass line and spectrophotometer areused for evaluation purposes in the examples set forth below; theseelements, and the associated valves, can be omitted from productionsystems.

In operation, the reaction cylinder R is filled with a borohydridereactant as discussed above, and the purifier cylinder P is filled witha hydroxide-containing reagent as also discussed above. The completesystem is evacuated through waste connection W and the associated pump.While reaction cylinder R is maintained at the desired reactiontemperature, and while purifier P is maintained at the desiredcontacting temperature, gas cylinder S and pressure regulating valve V0are operated to admit boron trifluoride into reaction cylinder R viavalve V1. The BF₃ reacts with the borohydride in cylinder R to yield amixture including diborane and BF₃. The outlet valve V2 and the purifiervalves V3 and V4 remain open, so that the mixture from cylinder R passesthrough purifier P to form purified diborane, which passes to thecollecting cylinder or using device F. Some of the purified diborane isdiverted through valve V7 to manifold M and IR spectrophotometer I. Tomonitor the composition of the mixture from reactor R, valves V3, V4 andV5 are shut, whereas valve V6 is opened to divert the mixture aroundpurifier P.

Using a potassium borohydride reactant in reactor R, the optimumconversion of BF₃ to diborane occurs at a reaction temperature of about−120 to about 30° C. When the reaction is carried out at highertemperature, a higher concentration of boron trifluoride in thediborane/boron trifluoride mixture resulting from the reaction isobserved. Similarly the boron trifluoride content in the mixture ishigher when sodium borobydride is used in place of potassiumborohydride. Because the hydroxide in purifier P effectively scavengesboron trifluoride from the mixture, sodium or lithium borohydride can beused in reactor R while still maintaining a high-purity diborane outputfrom the purifier. This is less preferred because the purifier capacityis reduced. In the purifier, optimum selective scavenging is achievedwith the use of lithium, sodium or potassium hydroxide. The mostpreferred hydroxide for the purifier is lithium hydroxide. As mentionedabove, the purifier P desirably is used at below room temperature, andmore preferably at dry ice temperature.

ILLUSTRATIVE EXAMPLES

The following Examples illustrate certain features of the invention:

Example 1

50 grams potassium borohydride was packed under helium atmosphere insidea glove bag into a cylindrical stainless steel reactor R (volume 195 cc)with flanges on each end. The reactor was closed with mounting flangeswith valves closed. This reactor assembly was placed within a jacketedempty cylindrical container. The purifier P was a stainless steelbubbler (972 cc volume) with a dip tube, welded top with inlet andoutlets valves with VCR fittings. The purifier was more than half filledthrough a fill port with potassium hydroxide pellets. The operation tofill the purifier was conducted inside a glove bag with helium flowing.The fill port was closed with ½″ VCR cap. The reactor and purifier wereconnected as illustrated in FIG. 1. All sections of the set up includingreactor and purifier were evacuated. The purifier P was gently heatedwhile evacuating to dry the potassium hydroxide. The jacketed containeraround the reactor was filled with dry ice. The purifier was cooledusing a Dewar filled with dry ice around it.

Boron trifluoride was admitted to reactor R from gas cylinder S with thepressure regulating valve regulator adjusted to maintain 1100 torr inletpressure to the reactor. The inlet valve V1 of the reactor was closedand outlet valve V2 opened to pass a sample of the reacted mixturethrough purifier P. A sample passing through purifier P at an outletpressure of 24 torr was collected in a pre-evacuated IR cell I. The IRscan was taken on Buck Scientific IR spectrophotometer. FIG. 2a showsthe IR spectrum of the sample; it indicates pure diborane. The IR cellwas brought back to manifold and evacuated. Both valves V3 and V4 on thepurifier were closed. A further sample of the mixture from the reactorwas collected in the IR cell at a pressure of 19 torr by opening theoutlet valve V2 of the reactor and bypass valve V6 to the bypass line B.The IR spectrum of the sample is shown in FIG. 2b. This spectrum showsdiborane and also shows absorption at 1450 cm⁻¹ characteristic of BF₃.These results indicate that the purifier has successfully removed BF₃from the mixture while leaving diborane substantially intact.

Example 2

The procedure of Example 1 was substantially repeated, except that thepurifier P with potassium hydroxide was kept at room temperature. Asample of passing through the purifier was collected at a pressure of 53torr, and the IR scan was taken. The spectrum shows only small amount ofdiborane and no indication of BF₃. However another sample was collectedat 14 torr through bypass line B, thus bypassing purifier P, containeddiborane and some boron trifluoride; the spectrum of this sample wassimilar to FIG. 2b. These results suggest that at room temperature thepurifier removes boron trifluoride completely, but somedisproportination of diborane occurs.

Example 3

The procedure of Example 1 is substantially repeated, except that thepurifier was filled with soda lime (a mixture of sodium hydroxide,calcium oxide and calcium hydroxide). The reactor and purifier werecooled to dry ice temperature. A sample from the reactor passing throughthe purifier was collected at 51 torr, and another sample was collectedbypassing the purifier at 19 torr. Comparison of the IR scans for thesesamples confirmed that purifier scrubs the boron trifluoride from thediborane/boron trifluoride mixture. With soda lime, however, somedisproportionation of diborane into non-condensable hydrogen wasobserved.

Comparison Example 4

Reactor R used in Example 1 was cleaned, dried and then filled with 62.4grams of sodium borohydride and attached to the set up of FIG. 1 and tothe vacuum manifold. The purifier P used in Example 1 was filled with247 grams of lithium hydroxide. The purifier P used in Example 1 washeated to 60° C., repeatedly purged with helium and evacuated. Thejacket of the reactor R and Dewar of purifier P were filled with dry iceand allowed them to cool to dry ice temperature. Boron trifluoride wasadmitted to the reactor at 760 torr pressure. The inlet valve V1 of thereactor was closed and the outlet valve V2 was opened to the bypass lineB and manifold. A sample was collected at 50 torr was collected in thepre-evacuated IR cell. FIG. 3b shows the IR scan of the sampleindicating diborane and a significantly higher content of unreactedboron trifluoride than that found in Example 1 with potassiumborohydride.

Example 5

The IR cell was again evacuated and a 22.4 torr sample was collectedfrom the reactor of Comparison Example 4 by opening the reactor outletthrough purifier P. The resulting IR scan, shown in FIG. 3a, shows onlydiborane and shows the absence of any boron trifluoride.

Comparison Example 6

The procedure of Comparison Example 4 was substantially repeated, exceptthat the reactor R was kept at room temperature. Boron trifluoride wasopened to the reactor at 800 torr pressure and the inlet valve closed. Asample of 50 torr was collected in the IR cell, bypassing the purifier.The IR spectrum showed predominantly boron trifluoride, indicating thatlittle if any diborane had formed.

Example 7

The procedure of Comparison Example 6 was repeated, but using a reactorcontaining potassium borohydride. The IR spectrum indicatessignificantly higher content of diborane than that achieved inComparison Example 6.

Example 8

Conversion efficiency and diborane yield from reaction of BF₃ andpotassium borohydride were determined. 7.75 grams (0.1437 moles) ofpotassium borohydride was loaded in a 75 ml stainless steel samplecylinder inside a glove bag with helium atmosphere. The cylinder wasclosed with a diaphragm valve and mounted on a vacuum manifold. Borontrifluoride was transferred into the cylinder and condensed therein insix different attempts. The cylinder was cooled with liquid nitrogen.The amount of boron trifluoride transferred varied from 0.01 to 0.077moles in these attempts. Each time boron trifluoride was transferredinto the sample cylinder was weighed and stored at 0° C. in a freezer.The reaction mixture present in the cylinder was analyzed at 30 torreach time by IR scan. After analysis the product was transferred andsample cylinder weighed and refilled with boron trifluoride. In each ofthe six attempts complete reaction yielding diborane was observed thoughIR scan. A total of 0.19 mole boron trifluoride completely reacted toyield diborane. In the next attempt when 0.01 mole of boron trifluoridewas added and left over a week in the freezer, the product mainlycontained boron trifluoride indicating exhaustion of potassiumborohydride. The experiment revealed that 4 moles of boron trifluoridereacts completely with 3 moles of potassium borohydride to yielddiborane.

Example 9

In this experiment, 21.1 grams potassium borohydride was taken in a 175ml stainless steel cylinder (reactor cylinder) then closed with astainless steel diaphragm valve. The cylinder was evacuated and weighedand mounted back on the vacuum manifold. Boron trifluoride 8.2 grams(0.1209 moles) was condensed into this cylinder at liquid nitrogentemperature and was placed in the freezer at −40° C. for a week. Thecylinder was taken out of the freezer and mounted at the inlet side ofthe purifier containing lithium hydroxide. The outlet side of thepurifier on the vacuum manifold was connected to a pre-evacuated 175 mlstainless steel cylinder (receiver). The purifier was cooled with dryice and the receiving cylinder was cooled with liquid nitrogen. Thevapor (diborane) from the above cylinder was transferred passing throughthe purifier and into the cold receiving cylinder. A total of 1.6 gramssample, noted by the weight loss in the reactor cylinder, wastransferred. The weight gain of the receiving cylinder was also observed1.6 grams. The analysis of the sample revealed pure diborane with CO₂impurity less than 10 ppm. The experiment thus shows the efficacy of thepurifier in removing carbon dioxide impurity. The purifier in theexample would also effectively eliminate any higher boranes.

As the foregoing and other variations and combinations of the featuresdiscussed above can be used without departing from the invention asdefined by the claims, the foregoing description of preferredembodiments should be taken by way of illustration, rather than by wayof limitation, of the invention as defined in the claims.

What is claimed is:
 1. A process for removing carbon dioxide from amixture of carbon dioxide and an inorganic hydride comprising the stepof contacting said mixture with a reagent composition comprising one ormore inorganic hydroxides.
 2. A process as claimed in claim 1 whereinsaid inorganic hydride consists essentially of diborane.
 3. A process asclaimed in claim 1 wherein said inorganic hydride is selected from thegroup consisting of diborane, silane, germane, phosphine, arsine,stibine and mixtures thereof.
 4. A process as claimed in claim 1 whereinsaid reagent composition consists essentially of said one or morehydroxides.
 5. A process according to claim 1 wherein the reagentcomposition comprises one ore more alkali metal hydroxides.
 6. A processaccording to claim 5 wherein the reagent composition comprises lithiumhydroxide.
 7. A process according to claim 5 wherein the reagentcomposition comprises sodium hydroxide.
 8. A process according to claim5 wherein the reagent composition comprises potassium hydroxide.
 9. Aprocess according to claim 1 wherein the reagent composition comprisesone ore more alkaline earth hydroxides.
 10. A process according to claim1 wherein the reagent composition comprises ammonium hydroxide.
 11. Aprocess according to claim 1 wherein the reagent composition comprisesone ore more transition metal hydroxides.
 12. A process according toclaim 1 wherein said reagent composition is at a temperature below about0° C. during said contacting step.
 13. A process according to claim 12wherein said temperature is between about −92° C. and −40° C.
 14. Aprocess as claimed in claim 13 wherein said temperature is about −80° C.15. A process as claimed in claim 1 further comprising the step ofutilizing the purified inorganic hydride at a consumption location, saidcontacting step being performed at said consumption location.
 16. Aprocess according to claim 15 wherein said contacting step is performedconcomitantly with said utilizing step so that said inorganic hydride ispurified and used within about 4 hours after it is purified.
 17. Aprocess as claimed in claim 16 wherein said inorganic hydride is usedimmediately after it is purified.
 18. A process according to claim 1wherein said contacting step is performed by passing said mixture oversaid reagent composition, said reagent composition being in a solidphase.
 19. A process as claimed in claim 18 wherein said contacting stepis performed at a pressure below the equilibrium vapor pressure of saidinorganic hydride at the temperature prevailing in the purifier.
 20. Aprocess as claimed in claim 18 wherein said contacting step is performedby passing a continuous flow of said mixture through a vessel containingsaid reagent composition.
 21. A process as claimed in claim 18 whereinsaid composition is in the form of powder, pellets, or granules.
 22. Aprocess as claimed in claim 1 wherein said composition is in the form ofa coating on a substantially inert support.
 23. A process according toclaim 1 further comprising the step of pretreating said reagentcomposition by heating said reagent composition in a substantially inertatmosphere.
 24. A process for purifying a material containing one ormore inorganic hydrides and containing carbon dioxide as an impurity,the process comprising contacting the material with a compositionincluding one or more hydroxides selected from the group consisting ofalkali metal hydroxides, alkaline earth hydroxides and ammoniumhydroxide to thereby remove said carbon dioxide.
 25. The processaccording to claim 24, wherein the contacting step is performed at atemperature such that said inorganic hydride is substantially retainedin the material and said carbon dioxide is substantially removed fromthe material.
 26. The process according to claim 24, wherein saidinorganic hydride consists essentially of diborane.